The invention relates to a method and system in connection with permanent magnet synchronous machines (PMSM). More specifically, the invention relates to a method of determining the angular speed and the rotor position of a speed and position sensorless PMSM drive equipped with an output filter and to an apparatus for carrying out the method.
Problems may be encountered in AC motor drives due to the non-sinusoidal voltage produced by a pulse-width modulated (PWM) inverter. The high rate of change of the voltage (i.e. high du/dt) may cause excessive voltage stresses in the stator winding insulations. It may also excite the parasitic capacitances of the stator winding and produce bearing currents. Lower-order harmonics cause acoustic noise and power losses; the losses caused by eddy currents are a special concern in high-speed solid-rotor motors.
A common approach to overcome these problems is to use an inverter output filter [1]-[4]. An LC filter, having the resonance frequency below the switching frequency, is a typical choice for the filter topology if a nearly sinusoidal output voltage is required. If a conventional scalar control method is used, the heavy filtering of the LC filter does not complicate the drive control. When better dynamic properties are demanded, a vector control method must be used. For vector control, the filter dynamics should be taken into account in the control design.
Various methods have been proposed for the vector control of variable-speed drives equipped with an LC filter [2]-[8]. Methods based on a feed-forward action and a sliding mode control are proposed for compensating for the effects of the filter in a speed-sensorless permanent magnet synchronous motor (PMSM) drive [2]. A full-order observer for a PMSM is implemented in the stator reference frame for estimating the rotor position in [3]. A feed-forward controller is used in a high-speed synchronous reluctance motor drive with an LC filter in [4]. In these methods, stator current or stator voltage measurements are needed. Vector control methods for induction motor drives with an LC filter are proposed in [5, 6] also require measurements from the motor side of the filter.
Due to the LC filter, the electrical quantities of the motor input differ from those of the inverter output. Frequency converters are equipped with measurements of the inverter output electrical quantities, but the motor terminal quantities are needed for vector control. In [2]-[6], the motor voltages or currents are measured by additional sensors, requiring hardware modifications in the motor drive. If the motor quantities are estimated instead, as proposed in [7, 8] for induction motor drives, the additional measurements are avoided and a filter can be added to an existing drive.